One conventional fiber-type amplifier, such as an erbium doped fiber amplifier (EDFA), uses an optical fiber doped with a rare earth element. To achieve favorable transmission properties, the gain wavelength property G (λ) of a fiber-type amplifier has to be flat to achieve favorable transmission properties.
The gain wavelength property G (λ) of an optical amplifying medium, such as an erbium doped fiber (EDF), varies according to the environmental temperature (for example, −15 [degrees C.] to +60 [degrees C.]) and may consequently drop. In contrast, a technique is known of adjusting the temperature of the optical amplifying medium using a temperature adjusting unit (see, e.g., Japanese Laid-Open Patent Publication No. H4-11794).
Nonetheless, according to the conventional technique, a problem arises in that the size of the apparatus is increased because a long optical fiber is used in the fiber-type amplifier.